1. Field of the Invention
The present invention relates to a power zoom lens having an automatic focusing device and a camera body and camera system having such a power zoom lens.
This application is related to the commonly assigned applications U.S. Ser. Nos. 07/410,880, 07/652,038, 07/407,424, 07/414,477, 07/414,478, 07/670,733, 07/406,523, and 07/414,476, the disclosures of which are expressly incorporated by reference herein.
2. Description of Related Art
There are various types of known lens shutter type cameras having power zoom lenses. In a known lens shutter type of power zoom camera, having no interchangeable lens, the power zooming operation is controlled by a control device and a zooming switch, provided in a camera body. A lens shutter type camera having a power zoom function is easily realized, since the lens is integral with the camera body.
However, it is very difficult to provide a camera having an interchangeable lens, such as a single lens reflex camera, with a power zooming function. Incorporating a power zooming function into an interchangeable lens would complicate the internal construction thereof. This is one of the main reasons that an interchangeable lens having a power zooming function has not been marketed.
Furthermore, it is necessary to detect the position of a focusing lens group, and the position of a zooming lens group, in order to obtain object distance data and focal length data, which is used to determine the photographic requirements for taking a picture at an optimum exposure.
In a known power zoom lens camera having an automatic focusing device, a detecting device for detecting the positions of the focusing lens group and the zooming lens group is, for example, formed by code plates which detect absolute positions of the lens groups. For instance, in a conventional power zoom lens, the focal length is detected by a code plate which is secured to a zoom ring which rotates about an optical axis or moves along the optical axis in accordance with the zooming operation. The code plates extends in the direction of movement of the zoom ring. A code reading member is provided on an immovable portion of the power zoom lens and is opposite the code plate, so that the codes of the code plate are read by the code reading member to detect the focal length. The position of the focusing lens group can be similarly detected.
However, the detecting device, which is made of code plates, can detect the positions of the lens groups corresponding to switching points of the codes, but it cannot detect the positions corresponding to the intermediate points between the switching points of the codes. Namely, only a rough detection (measurement) can be made, i.e., precise detection (measurement) cannot be effected.
Another known manner in which the detecting of lens group positions can be carried out is to count the number of pulses (i.e., add or subtract the number in accordance with the direction of movement of the lens groups) outputted from a pulse generator in association with the rotation of a motor, which drives the lens groups, in order to detect the positions of the lens groups. However, in this manner of detecting, an error inevitably occurs in the number of pulses counted by the counter. To avoid this error, in a conventional camera, the pulse number is corrected when the lens groups reach an infinite extremity, a macro extremity, a wide angle extremity, a telephoto extremity, etc.
However, since the correction can only be made when the lens groups reaches one of the extremities, if the lens groups are moved several times within the range defined by and between the extremities, the error can become exceedingly great, resulting in an inaccurate detection.
If precise detection of the positions of the lens groups (object distance and focal length) cannot be made, zoom control, in which the object distance and the focal length are used as control parameters, cannot be carried out. For instance, a constant image magnification zooming operation, in which the magnification of an image determined by the object distance and the focal length is kept constant, cannot be effected.